Tuesday's planned protest outside the Malaysian parliament was the second by a coalition group known as Bersih ("clean" in the Malay language) demanding electoral reforms.
The group's first rally on November 10 drew some 30,000 people to the streets of Kuala Lumpur according to organisers before it was broken up by police using tear gas and chemical-laced water cannon.
Authorities put the number who attended the rally at less than 5,000.
November 10 – Organisers say 30,000 people join a rally in Kuala Lumpur calling for electoral reforms
November 25 – Tens of thousands of ethnic Indians attend a series of protests in and around Kuala Lumpur complaining of racial discrimination
December 9 – About 40 people led by lawyers hold a peace walk through Kuala Lumpur on the eve of Human Rights Day
Ahead of Tuesday's protest outside parliament authorities had obtained a court restraining order before declaring the planned gathering unlawful.
Malaysian laws forbid public gatherings of more than five people without a police permit.
Nonetheless Hatta Ramli, an official in PAS, the Islam-based opposition party, said there was no basis for the arrests.
"There is no chaos, no riot, nothing," he said.
Five activists who managed to slip into parliament while travelling in cars with opposition legislators distributed copies of a protest note to legislators before they were also arrested.
The note criticised a government-sponsored amendment bill that sought to extend the tenure of the Election Commission chief ahead of national polls next year.
The opposition has long maintained that the election chief is biased, accusations the government has dismissed as baseless.
Also on Tuesday police arrested P Uthayakumar, a leader of the Hindu Rights Action Force, a group that organised a November 25 rally of ethnic Indians in Kuala Lumpur to protest against alleged racial discrimination and economic deprivation.
Uthayakumar was later reported to have been charged with sedition.
The government led by Abdullah Ahmad Badawi, the Malaysian prime minister, has repeatedly warned that illegal public assemblies could disrupt the country's fragile racial balance.
Speaking to business leaders on Monday, Abdullah said he would defend the stability of the country and was willing to sacrifice public freedoms in order to do so.